1. Field of the Invention
The present invention relates generally to overhead projectors and, in particular, to projection lenses used with said projectors.
2. Description of the Related Art
Overhead projectors are used to form an enlarged image of a photographic transparency or other material on a projection screen. Overhead projectors generally include a base or housing which contains a light source and associated optical elements. A vertical beam of light is directed upward from the light source and through a transparent plate, or projection stage, located at the top of the housing. With conventional overhead projectors, a Fresnel condenser lens assembly located within the housing beneath the projection stage serves to concentrate light from the light source through the transparency and to concentrate light on a projection lens disposed above the projection stage.
Conventional overhead projectors also include a projection head which carries the projection lens assembly. The projection head is carried by a bracket which is slidably positionable along a vertical post extending upwardly from the housing. The most conventional form for overhead projection lenses is a two-element projection lens assembly utilizing two spaced positive meniscus lenses, one to direct light toward a mirror reflector, and the other to receive light from the mirror to project an image on a vertical projection screen or surface to enable the magnified image of the transparency to be viewed. The projected image is focused by movement of the projection lens assembly toward and away from the stage upon which the transparency to be projected is placed. Conventional two-element projection lenses are illustrated and described in such publications as U.S. Pat. Nos. 3,126,786 issued to Appeldorn and 3,334,957 issued to Field.
Conventional projection lenses for use in overhead projectors have a focal length in the range of 275 to 355 millimeters. The focal length of the projecting lens is determined by the magnification desired and the distance of the projector from the screen. For the magnification range through which this type of projection lens is used, the projection distance between the overhead projector and the projection screen must be several feet in order to project an image which is large enough for readability. For example, using a typical 350 millimeter focal length projection lens at a 5.7.times. magnification, the projector is between seven (7) to eight (8) feet (2.13 to 2.44 meters) from the screen.
However, this projection distance is often incompatible with smaller conference rooms. In smaller conference rooms, the overhead projector may not allow enough space between the presenter and the audience. This distance between the projector and the screen also may result in partial blockage of the screen by the projector and/or the presenter. It is therefore desirable in certain situations to be able to position the overhead projector closer to the projection screen.
A shorter focal length, "wide-angle" projection lens is one attempt to decrease the projection distance while maintaining the same image magnification. The most useful shorter focal length lenses for this application are in the 235 to 245 millimeter range. However, the use of this type lens results in a reduced back focal distance, i.e., the distance between the transparency and the center of the bottom lens. This type of short focal length lens is therefore incompatible with the focus adjustment range of conventional overhead projectors, because the focusing post of the overhead projector must be shortened significantly, and special Fresnel condensing optics must be used. Most importantly, these type of lenses have a reduced field coverage which will not image a full-sized A4 stage, so that an outer portion of the transparency will not be projected on the projection screen.
One way to increase the lens field coverage and back focus relative to the focal length is a lens system of the retrofocus or inverted telephoto type. With this type of lens, the back focus can be considerably increased with respect to the focal length. Although there have been many designs for lens systems of this type, e.g., U.S. Pat. Nos. 4,145,117 issued to Ikeda; 4,182,549 issued to Kreitzer; and 4,203,653 issued to Mori, these lens systems have not addressed the special requirements of overhead projection. Also, these lens system designs are often complex, consisting of five or more lens elements.
The present invention addresses the problems associated with the above projection lens designs.